1. Field of the Invention
This invention relates generally to melting or melting/holding furnaces and more particularly, it relates to a control system for reverberatory aluminum melting and melting/holding furnaces which includes means for measuring heat absorbed by the refractories so as to rapidly bring the furnace to the desired metal temperature without any significant overshoot.
2. Description of the Prior Art
As is generally well-known in the heat-treating furnace art, the bringing of a heat-treat metal workload such as aluminum stock materials and the like in a melting or melting/holding furnace to the desired metal processing temperature without any significant overshoot has always presented problems. The heat treating cycle requires that the aluminum material be raised to a fairly specific temperature such as 1400.degree. F. and soaked at that preselected processing temperature for a substantial period of time. It is extremely important that the aluminum material not be heated higher than the target temperature since various types of deterioration occur at these elevated temperatures. For example, such overshoot in the homogenizing of aluminum billets would result in excessive, undesirable grain growth in the aluminum. Further, the higher temperatures may also cause additional metal loss and promote the formation of dross.
A considerable amount of development has already taken place in the design of furnace-heating control systems for rapidly bringing the metal workload up to the processing temperature without any undesirable overshoot. These prior art control systems have not, however, employed thermocouple control of the heating chamber since a thermocouple extending into the firing chamber, either through the furnace sidewalls or roof, would be prone to damage by charging and cleaning practices in the heat-treating furnace art. Such conventional control systems generally include: (1) a flue thermocouple, (2) a roof overtemperature thermocouple embedded in the brickwork but not exposed directly to the furnace chamber, and (3) a bath thermocouple which is retracted in the furnace wall until a molten bath condition is achieved.
In operation of the prior art control systems, after charging of the furnace with the cold aluminum material the furnace is then operated with its gas-fired burners at "high fire" condition until the pre-set flue temperature has been reached as sensed by the flue thermocouple. The burners of the furnace are maintained in the "high fire" condition until the bath thermocouple is inserted into the molten metal for measuring its temperature. When the pre-set temperature has been reached, the burners are switched to a "low fire" or off condition.
However, the furnace is lined with high density-refractory materials which absorb heat during the burner firing cycle, and such refractory materials are exposed to temperatures up to 1,000.degree. F. higher than the final desired aluminum material. While the burners are switched to the "low fire" or off condition at the end of the firing cycle, the stored heat in the refractory materials will continue to radiate heat into the molten bath so as to produce a significant overshoot in the temperature. Thus, the energy absorbed by the molten bath is lost and it is therefore necessary to reduce the aluminum temperature to the desired value, either in the furnace or after transferring it to a holding furnace prior to casting.
It would therefore be desirable to provide an improved control system for reverberatory aluminum melting and melting/holding furnaces which includes means for measuring the heat absorbed by the refractories so as to rapidly bring the furnace to the desired metal temperature without any significant overshoot.
A state of the art search directed to the subject matter of this application uncovered the following U.S. Pat. Nos.:
U.S. Pat. No. 3,610,045 PA1 U.S. Pat. No. 4,324,129 PA1 U.S. Pat. No. 4,534,663 PA1 U.S. Pat. No. 4,567,849
There is disclosed in U.S. Pat. No. 3,610,045 to Wilbur E. Shearman issued on Oct. 5, 1971, an apparatus for obtaining continuous temperature measurement in a bath of molten iron. The apparatus includes a thermocouple disposed within a hollow ceramic refractory protective sheath having a portion thereof protruding within the melt. The wall thickness of the sheath is relatively thin at the tip and is of relatively greater wall thickness at the intersection of the sheath with the "hot face" of the furnace wall. The sheath is embedded into the furnace wall for a substantial length so as to provide the unit having a greatly increased life and a very accurate temperature measurement.
There is disclosed in U.S. Pat. No. 4,324,129 to Hiroshi J. Goldsmid issued on Apr. 13, 1982, a thermal comparator for use in identifying material such as gemstones which comprises a thermocouple to be applied to a material that includes two dissimilar metals and having junctions spaced relatively close to each other and forming thermocouple branches and a high thermal conductivity tip at one of the junctions. One branch is connected to a source of heat at least up to just prior to making contact between the thermocouple and the material to be tested so that the one branch forms the main heat source for at least a short time after the tip of one of the junctions is placed in contact with the material to be identified.
In U.S. Pat. No. 4,534,663 to Heinz F. Poppendiek issued on Aug. 13, 1985, there is taught an apparatus for measuring or comparing the quality of different heat insulation materials which includes a housing structure for confining two different insulation materials in corresponding rectangular cavities and on opposite sides of a special flat heat source.
In U.S. Pat. No. 4,567,849 to Chang-Feng Wan issued on Feb. 4, 1986, there is disclosed a differential thermal analysis apparatus which includes a first thermocouple 24 disposed within the reactor and a second thermocouple 30 disposed within a neutral body 16. The differential voltage between the two thermocouples 24 and 30 is sensed to provide a DTA measurement of transformation points of the melt 18.
However, none of the prior art uncovered in the search disclosed a melting/holding furnace like that of the present invention which includes a first thermocouple element disposed adjacent the "hot face" of the furnace wall layer and a second thermocouple element disposed adjacent the "cold face" of the wall layer.